The present invention relates to an image compression method and an image processing apparatus. More specifically, the present invention relates to an image compression method and an image processing apparatus that are suitable for compressing still image data of a camera, recorder, or other digital device capable of recording/reproducing both still images and motion images.
An image pickup processing apparatus disclosed in JP-A-2006-311347 can process a motion image and a still image in parallel without lowering the motion image frame rate when capturing a still image during motion image recording.
When a motion image and a still image are to be encoded in a motion image/still image camera, a technology disclosed in JP-A-2003-264834 performs encoding while the number of tiles into which the motion image is divided differs from the number of tiles into which the still image is divided.
In recent years, video cameras and other similar apparatuses capable of recording both motion images and still images have been widely used. The still images have increased resolution so that their sizes are larger than image sizes provided by motion image compression. These video cameras need to incorporate a motion image encoder and a still image encoder. However, motion image encoders do not generally support high-resolution still image sizes.
Meanwhile, the H.264 technology has attracted attention as a compression technology for motion image encoding. This technology offers approximately three times the degree of compression provided by a former motion image encoding technology, and will become a future standard for video cameras. For still images, however, the JPEG (Joint Photographic Experts Group) technology is still widely used. The JPEG technology is inferior to the motion image compression technology in image compression capability.
A method disclosed under the above circumstances compresses a still image after dividing it into segments in a situation where the employed encoder has a low image compression capability. For example, a method disclosed in JP-A-2006-311347 encodes a motion image as a Motion JPEG image with reference to the VGA size, and divides a still image as appropriate and compresses the resulting image segments as JPEG images (see paragraphs 0123 to 0125).
As shown in FIG. 12, a method disclosed in JP-A-2003-264834 reduces the number of tiles into which a motion image is divided, and reduces the number of tiles into which a still image is divided. This makes it possible to process the still image without sacrificing image quality (see paragraph 0057) and transmit the motion image at high speed for enhanced motion smoothness (see paragraph 0058).
It is demanded that a video camera capable of handling both still images and motion images should not incorporate both a motion image encoder and a still image encoder because the apparatus will have a complex configuration and become costly. Further, when the image format of compressed motion images differs from that of compressed still images, complicated file management needs to be exercised within the apparatus.
As such being the case, it is preferred that only one type of encoder be used. When, for instance, the H.264 encoding, MPEG, or other motion image compression standard is to be employed, technologies for dividing a still image and subjecting the resulting image segments to motion image compression may be used as described in JP-A-2006-311347 and JP-A-2003-264834.
However, the above-mentioned encoding technologies for motion image compression do not support a method of encoding still images and compressing them with high efficiency. The above-mentioned encoding technologies for motion image compression support a viewpoint of inter-frame time prediction and define different picture types (I picture, P picture, and B picture) for individual frames to increase the compression ratio. However, a viewpoint of time is not supported for still image compression. The above-mentioned conventional technologies are not developed to increase the overall image compression ratio while avoiding image quality deterioration in an important portion of a still image.
The present invention has been made to solve the above problem. An object of the present invention is to provide an image compression method that exhibits a high overall image compression efficiency while avoiding image quality deterioration in an important portion of a still image by using a motion image compression technology for still image compression in an apparatus capable of handling both motion images and still images.